The Gospel of Thomas claims to be the secret sayings of Jesus. There are 114 of them, so it says many things, but the central message is that Jesus is the one who reveals the divine light that brought the universe into being, and that you and I also reveal that light.
What's different about the Gospel of Thomas is that, instead of focusing entirely on who Jesus is and the wonderful works of Jesus, it focuses on how you and I can find the kingdom of God, or life in the presence of God.
We don't actually know if the person who wrote the Gospel of John had a written copy of Thomas because we don't know exactly when it was written.